1. Field of the Invention
The subject invention generally pertains to methods of removing pipe from a wellbore, and more specifically to devices used in explosively splitting a coupling longitudinally, and still more specifically to charge assemblies that are used in such devices.
2. Description of the Related Art
It is often desirable to sever, split or otherwise cut a string of tubing or casing to recover the pipe from wellbores. In cutting pipe within a wellbore, pipe restrictions are often encountered. These restrictions may be in the form of a packer or fishing spear placed within the pipestring for the purpose of retrieval, or they may be of natural causes such as scale, paraffin, collapsed pipe, or smaller inner string of pipe stuck within the larger diameter pipe that is to be cut. Restrictions inhibit the use of present cutters that require a full opening or full inside diameter to achieve an effective cut. Other folding or spring-loaded devices have been developed to run through these restrictions, but these devices have met with little commercial success due to their mechanical complexity and high failure rate.
Over the years a variety of methods for cutting pipe in a wellbore have been developed. Some of these include chemical cuts, backoff shots, nitroglycerin, and various forms of shaped charges.
Chemical cuts are extraordinarily expensive and require the outer edge of the cutting device to be immediately adjacent (within a fraction of an inch) to the pipe being cut. By its design, the outer diameter of the chemical cutter head must be very close to the inside diameter of the pipe being cut. This limits the use of the chemical cutter in tubulars that have a restriction above the cutting point. Due to the "piston effect", the cutter floats into the hole, thereby slowing down the costly process of cutting and retrieving pipe from the ground.
Backoff shots are another way of separating the pipe within a wellbore. This process is simply placing an explosive device across a coupling and putting left-hand or reverse torque the string of pipe to be backed off. When the proper reverse torque is in the pipe, the explosive is discharged thereby creating shock waves at that point. The pipe then simply unscrews. The limitation of this method of pipe retrieval is that there is no guarantee as to where the pipe might unscrew.
The use of nitroglycerin is another method of severing the pipe at a coupling. This method, although simple and economical, simply blows up the tubulars and its immediate environment. Better said, it makes a mess of the pipe that is pulled and left in the ground. The use of nitroglycerin is not environmentally sound in that it prohibits or limits the reentering of this wellbore for future use.
There are various forms of radial-shaped charges in use and several of these products offer excellent cuts, however they have two inherent problems. As in the chemical cutter, the outside diameter of the radial cutter assembly must be very close to the target or inside the diameter of the pipe being cut. This design limitation is due to the shaped charge design phenomenon of "standoff" whereby the distance between the charge and the target is crucial to its performance. Another resultant problem resulting from the large outside diameter of the cutter is that it has a "floating effect" as it is lowered into the hole. Additional weights are required to help push it into the hole. By-in-large though, the biggest drawback to the use of the radial charge is that it cannot be run through any significant restriction or constriction in the pipe. In other words, one must have a full opening from the surface to the required cutting depth.
The remaining option for cutting downhole tubulars is the use of the linear-shaped charge. As in the radial charge, the standoff phenomenon has dictated the design of various devices using the linear form of a shaped charge. Several of these devices use mechanical springs, unfolding charges or remotely extendible frameworks to properly position the charge with the proper design standoff against the coupling to be cut. Again, the complexity of such mechanisms have proved to be unreliable and impractical when exposed to the severe pressures and temperatures of downhole environments.
The amount of explosive in the linear charges needed for effectively cutting, downhole pipes and couplings is generally too great to allow them to be shipped by air and economically shipped by ground transportation. This is due to certain government regulations that prohibit the air transportation of certain classes of explosives and place stringent requirements on those transported along roadways. As slower, more expensive, modes of transporting these explosives must be used, this increases the shipping time and costs and may often result in long and costly delays.